1. Field of the Invention
The present invention relates to a displacement measuring apparatus for calculating a movement distance of a probe brought into contact with an object in accordance with a change in oscillation frequency of an oscillator and, more particularly, to a displacement measuring apparatus for correcting a secondary curve so that the calculated movement distance is approximate to a true movement distance of the probe.
2. Description of the Related Art
A dial gauge shown in FIG. 1 is known as a portable displacement measuring apparatus for accurately measuring a very short movement distance, for example, 0.1 mm or 0.01 mm, of an object. Displacement measuring apparatus 1 represented by such a dial gauge digitally displays a movement distance of probe 2 from a reference position on display unit 3. The movement distance of probe 2 is detected as an electrical signal corresponding to a change of an oscillation frequency of an oscillator incorporated in displacement measuring apparatus 1. More specifically, as shown in FIG. 2, core member 4 is joined with a shaft for supporting probe 2, primary coil 5 and two secondary coils 6a and 6b which are wound around core member 4. Coils 5, 6a, and 6b are arranged as shown in FIG. 3. An AC signal having a predetermined frequency is applied to primary coil 5 from oscillator 7, so that inductance L can be detected from output terminals 8a and 8b of secondary coils 6a and 6b. Therefore, when core member 4 is moved up and down, inductance L between output terminals 8a and 8b is changed.
FIG. 4 is a schematic view of displacement measuring apparatus 1. Colpitts oscillator 9, as shown in FIG. 5, comprises inductor L between output terminals 8a and 8b, capacitor C1, variable capacitor C2, and transistor Tr. The relationship between change in oscillation frequency f of the oscillator 9 generated in accordance with movement of core member 4 joined with probe 2 through coils 5, 6a, and 6b, and movement distance D of core member 4 from a reference position (central position) is linear, as represented by a solid line shown in FIG. 6. Therefore, movement distance D can be calculated in accordance with change of oscillation frequency. Oscillation frequency f of oscillator 9 is counted by counter 10, and movement distance D is digitally calculated by process section 11 in accordance with transform characteristics as indicated by the solid line in FIG. 6. The calculation result is displayed on display unit 12.
The following problem occurs in the displacement measuring apparatus for calculating movement distance D of probe 2 in accordance with the change in oscillation frequency f of oscillator 9 generated in correspondence with movement of core member 4 joined with probe 2 through coils 5, 6a, and 6b. As described above, in order to accurately calculate movement distance D of probe 2 using the displacement measuring apparatus, the relationship between the true movement distance of core member 4 from the reference point and change of oscillation frequency f must be completely linear, as indicated by the solid line in FIG. 6.
However, the relationship between movement distance D of probe 2 and change of the oscillation frequency f is not linear due to disturbance in lines of magnetic field among coils 5, 6a, and 6b, an edge effect on both ends of core member 4, or the like, as indicated by a dotted curve in FIG. 6. FIG. 7 is an enlarged graph showing a relationship between true movement distance l of probe 2, i.e. core member 4 for D=0 and error (l-D) obtained by subtracting movement distance D calculated from true movement distance l. As shown in FIG. 7, error (l-D) is curvilinearly changed with respect to true movement distance l and maximum error reaches about 20 .mu.m (0.02 mm). Note that when a capacitance of variable capacitor C2 is changed, a gradient of a straight line in FIG. 6 is also changed, but the maximum error of 20 .mu.m in FIG. 7 is hardly changed.
This displacement measuring apparatus having maximum error of 0.02 mm satisfies a standard of a measuring apparatus for measuring a length. Therefore, this error is allowed in usual measuring processes.
When a measurement precision of about 1/100 mm in measurement for a vibration magnitude upon rotation of a rotation shaft of a high-precision component is required, however, the above error is not allowed.
As has been described above, it is required that a displacement measuring apparatus which can greatly improve the conventional measurement precision be developed.